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Particle Scattering

1. The Principle of Particle Scattering

When light passes through particles, light scattering occurs. The intensity distribution, polarization state, and spectral characteristics of scattered light are closely correlated with the properties of scattering particles. By detecting scattered light, abundant information regarding the microstructure and physical properties of particles can be obtained. Based on light scattering theory, multiple analytical technologies have been developed, including microparticle measurement, spectroscopy, rainbow refractometry, and phase Doppler anemometry. These techniques are widely applied in microparticle measurement systems such as particle size analyzers. Moreover, Raman spectroscopy and Brillouin scattering play essential roles in multiphase flow analysis and optical combustion diagnostics.

2. Principle and Components of Laser Particle Size Analyzers 

Its working principle is as follows: during laser propagation, the laser wavefront is constrained by apertures or particles with dimensions comparable to the optical wavelength. The elementary waves emitted from the constrained wavefront undergo spatial interference, thereby generating diffraction and scattering. The spatial (angular) distribution of diffracted and scattered light energy is dependent on both the light wavelength and the size of the apertures or particles.
For diffraction of particle groups, the quantity of each particle size class determines the magnitude of light energy obtained at the corresponding specific angles. The proportion of light energy at each specific angle relative to the total light energy reflects the abundance distribution of each particle size class.
Based on this principle, a mathematical and physical model correlating granularity abundance with the light energy acquired at specific angles can be established. Instruments are further developed to measure light energy. By comparing the light energy detected at a specific angle with the total light energy, the abundance ratio of corresponding particle size classes in the particle group can be calculated.
Laser particle size analyzers based on the Mie scattering principle assume measured particles to be ideal spheres and cannot characterize particle morphology; most of such instruments are offline models. They enable accurate measurement of particle size and distribution, covering a wide range from millimeters, micrometers, and submicrons down to the nanometer scale.
Laser particle size analyzers are classified into wet dispersion analyzers, dry dispersion analyzers, and integrated dry-wet dispersion analyzers according to their dispersion systems. There are also specialized types, such as spray laser particle size analyzers and online laser particle size analyzers.

3. The main components of a laser particle size analyzer

1. Laser light source
Lasers with good monochromaticity are commonly adopted, and single-frequency lasers with an ultra-narrow linewidth are the preferred choice.
2. Optical System
Beam expander : Expand the laser beam into parallel light, ensuring a uniform spot on the sample.
Lens assembly: Includes a Fourier lens or focusing lens, designed to collect scattered light and project it onto the detector.
3. Sample Dispersion System
Wet dispersion: Achieves uniform distribution of particles in a liquid using a circulation pump, an ultrasonic disperser (to prevent particle agglomeration), and a sample cell.
Dry dispersion: Uses compressed air or a vibration device to disperse dry powders.
4. Scattered Light Detector
Annular or multi-ring array detectors (such as photodiode arrays) are used to capture scattered light signals from different angles.
Forward small-angle detectors (for measuring large particles) and side/backward detectors (for measuring small particles).
5. Signal Processing and Control Systems
The optical-to-electrical converter transforms light signals into electrical signals, which are then amplified and transmitted to the computer.
The software calculates the particle size distribution using algorithms such as Mie theory or the Fraunhofer diffraction model.
6. Data Processing Software
Displays the distribution of scattered light energy in real time and fits a particle size distribution curve (typically expressed as volume percentages).

4. Laser Source for Laser Particle Size Analyzers

Lasers used in laser particle size analyzers typically require high power stability (<0.3%), wavelength stability (<±0.1nm), and ultra-low noise (<0.1% over 4 hours). They also demand high repeatability and beam quality (TEM00), along with strong environmental adaptability, compact, mechanically robust structure to ensure long-term thermal and mechanical stability. Shorter wavelengths yield higher measurement accuracy. Narrow-linewidth and single-frequency lasers are preferred, with wavelengths including 405 nm, 532 nm, 633 nm (as a replacement for He-Ne lasers), 671 nm, 780 nm, and 830 nm.